CHAPTER 5 VOICE COMMUNICATION CONCEPTS AND TECHNOLOGY Concepts Reinforced Top-down model Protocols and interoperability OSI Model Concepts Introduced Voice digitization Data/voice integration Voice compression Voice network concepts PBX functionality and architecture Computer telephony integration Voice transmission alternatives Voice over the Internet OBJECTIVES After mastering the material in this chapter you should be able to: • Understand the underlying technical concepts for voice transmission, voice digitization, voice compression, and data/voice integration. • Understand currently available voice-related technology including PBXs, voice digitizers, and voice/data multiplexers and modems. • Understand the functionality, standards, business impact, and technology involved with computer telephony integration. • Understand the functionality, concepts, standards, business impact, and technology involved with voice network services, voice transmission alter- natives, and voice/data integration. ■ INTRODUCTION Network analysts must be qualified to design networks that are capable of carrying voice as well as data. Before designing such networks, it is essential for the network analyst to understand the nature of voice signals, as well as how voice signals can be processed and integrated into a cohesive network with data transmissions. 164 Chapter Five/Voice Communication Concepts and Technology Once based exclusively on analog transmission, voice communication is rapidly becoming dependent on digital transmission technology. Once the voice signal has been digitized, a wide variety of transmission services can potentially be employed to complete the transmission of the voice signal to its designated destination. As is the case with any type of communications system involving the interoperability of multiple pieces of hardware and software technology, standards play an essential role in assuring end-to-end interoperability. Although traditional telephony continues to be the bearer of most voice calls, alternatives such as voice over IP (VOIP) promise equal quality at significantly lower cost. Technologies such as H.323 and SIP, combined with broadband Internet tech- nologies, have the potential to replace the modern telephone network architecture and revolutionize how calls are placed and billed. One of the fastest-growing areas of telecommunications is wireless telephony. With the development of higher capacity, better-sounding digital networks cellular phone use has grown exponentially in the United States and worldwide. In addition to voice communication, these networks are capable of carrying digital data to a spate of new handheld devices. As these wireless digital networks continue to evolve, new applications that take advantage of their better interoperability and higher data speeds will continue to be developed. A private branch exchange (PBX) acts as a local phone switch for an organiza- tion. The PBX allows for calls to be made inside the organization without using the PSTN, while allowing for calls to be made between the internal extensions and the PSTN. Newer PBX features allow for the integration of VOIP and wireless technolo- gies into the PBX. ■ VOICE TRANSMISSION BASIC CONCEPTS The modern telephone system is commonly known as the public switched telephone network, or PSTN. A voice conversation consists of sound waves that are of varying frequency and amplitude and are represented as a continuously varying analog waveform. The POTS (plain old telephone service) network employed analog trans- mission methodologies to transmit the voice signals from source to destination. But how does this analog waveform get from a person’s mouth, the human transmitter, onto the PSTN and subsequently into the ear, the human receiver, of the person who was called? Figure 5-1 illustrates the mechanics of a typical phone hand- set, which consists of both transmitter and receiver components. The telephone handset, consisting of both a transmitter and receiver, is really a fairly simple device that works largely based on the properties of electromagnetism. The transmitter, or mouthpiece, contains a movable diaphragm that is sensitive to changes in voice frequency and amplitude. The diaphragm contains carbon gran- ules that have the ability to conduct electricity. Because the human voice spoken into the transmitter varies, the amount of carbon granules striking the electrical con- tacts in the mouthpiece varies, sending a varying analog, electrical signal out onto the voice network. This constantly varying analog electrical wave is transmitted over the voice net- work to the phone of the receiving person. The receiver or earpiece portion of the handset basically works in the opposite fashion of the mouthpiece. The varying elec- trical waves produced by the transmitter are received at the receiver at an electro- magnet. Varying levels of electricity produce varying levels of magnetism that, in Voice Transmission Basic Concepts 165 Electromagnet Speaker diaphragm Receiver (movable) (earpiece) Sound Waves Permanent magnet Variable magnetic field Electrical contacts Handset Diaphragm (movable) Sound Waves Transmitter (mouthpiece) RJ-11 connectors 4 wires Granulated carbon RJ-22 connector 2 wires RJ-22 connector Figure 5-1 Getting Voice onto and off of the Network turn, cause the diaphragm to move in direct proportion with the magnetic variance. The moving diaphragm produces varying sound waves that correspond to the sound waves that were input at the transmitter. The electromagnetically reproduced sound produced at the receiver resembles the actual sound waves input at the trans- mitter closely enough to allow for voice recognition by the receiving party. Voice Bandwidth Although the approximate range of hearing of the human ear is between 15,000 to 20,000 Hz, significantly less bandwidth is used to transmit the electromagnetic repre- sentations of analog voice signals over the analog PSTN. POTS uses a bandwidth of 4,000 Hz including two guardbands to prevent interference from adjacent frequencies from interfering with the voice signal. As a result, the usable bandwidth on the local loop circuit connecting an individual’s home or business to the phone company’s central office for dial-up analog voice transmission is 3,000 Hz, from 300 to 3,300 Hz. 166 Chapter Five/Voice Communication Concepts and Technology Figure 5-2 illustrates the comparative bandwidths of human speech and the analog phone network. This limited bandwidth is why people sound less lifelike on the tele- phone than in person. ■ VOICE NETWORK CONCEPTS Telephone calls are connected from source via circuit switching. Circuit switching is an analog telecommunications term that originally meant that a physical electrical circuit was created from the source telephone handset to the destination telephone handset. In the early days of the telephone system a telephone system operator man- ually connected these connections at a switchboard. Later the rotary telephone and automatic switching was introduced. A better definition of a switched connection in the modern phone system is a reserved bandwidth connection between two telephone handsets. Although there is no longer a physical circuit in place between the handsets, the capacity on the tele- phone network required to deliver the call is reserved for the exclusive use of the call: The same amount of telephone system capacity is used by two people who are being perfectly quiet as is used by two people who are talking at the same time. The capac- ity is dedicated to the call as soon as it is placed. Basic Telecommunications Infrastructure Figure 5-3 illustrates the major components of the PSTN. The circuits between a resi- dence or business and the local central office or CO are known as local loops. A cen- tral office is a facility belonging to the local phone company in which calls are switched to their proper destination. As covered in chapter 1, the LATA is an area 20,000 4,000 Range of human hearing Guardband Hertz (Hz) 3,400 (graph not to scale) Full bandwidth Bandwidth of analog available for circuit analog voice transmission 300 20 Guardband 0 Figure 5-2 Voice Bandwidth Voice Network Concepts 167 Belongs to LEC (Local exchange carrier) C.O. Phone Phone Local loop Local loop trunk line LATA—B Inter-exchange LATA—A circuit P.O.P. trunk line P.O.P. C.O. Belongs to IXC (Inter-exchange carrier) Figure 5-3 Basic Telecommunications Infrastructure within which the local carrier completed all of the calls during the time period between the break-up of AT&T and the Telecommunications Act of 1996. Telephone calls are established by a device located at the local telephone com- panies CO known as a telephone switch. The telephone switch is directly connected to the customer’s telephone handset via the local loop. The telephone switch routes calls to the destination telephone handset. Requested destinations for phone calls are indicated to the telephone switch by dialing a series of numbers. These numbers tell the telephone switch whether the call will be local, intra-LATA, or inter-LATA, and subsequently, which circuits must be accessed and combined to complete the call as requested. All voice traffic destined for locations outside of the local LATA, and some traffic within the LATA, must be handed off to the long distance or inter-exchange carrier (IXC) of the customer’s choice. Competing long-distance carriers wishing to do busi- ness in a given LATA maintain a switching office in that LATA known as a POP, or point of presence. This